Magnetic tunnel junction (MTJ) sensors are a new type of magnetorestive sensor which has been used recently in various applications. It utilizes a magnetic multilayer and the magnetoresistance effect is called tunneling magnetoresistance (TMR). The TMR effect is related to the magnetic orientation of magnetization of the ferromagnetic layers making up the magnetic multilayer. TMR is advantageous since it is a larger magnetoresistance effect than that of commonly used technologies such as anisotropic magnetoresistance (AMR) effect, giant magnetoresistance (GMR) effect, or the Hall Effect, thereby producing a larger signal. When compared to the Hall effect TMR has better temperature stability. TMR has a further advantage of high resistivity and therefore can have very low power consumption. To summarize, MTJ devices have higher sensitivity, lower power consumption, better linearity, dynamic range, wider, better temperature characteristics than AMR, GMR, or Hall devices. The resulting noise performance of TMR devices is better. In addition, MTJ materials can be fabricated using existing semiconductor processes thereby providing the capability to produce MTJ sensors with very small size.
It is common to use a push-pull sensor bridge rather than a single sensor element in order to produce a magnetic field sensor, since bridge sensors have higher sensitivity, and an intrinsic temperature compensation function that suppresses drift. The traditional magnetoresistive push-pull sensor bridge requires two adjacent bridge arms in which the pinned layer magnetization is set in the opposite directions, in order to produce the push-pull effect. For low cost, it is preferable to deposit the sensor arms with opposing pinned layer magnetization direction onto the same silicon substrate. This however is not ideal for manufacturing, since there are no standard methods for setting the magnetization direction f the adjacent arms. They are usually set in the same direction. Present methods for producing push-pull bridge magnetoresistive sensors include double-deposition in which different films with different pinned layer magnetization set directions are deposited. Manufacturing is however difficult, since it is difficult to match the bridge legs, and annealing of one leg may alter the performance of the other.
Multi-chip packaging (MCP) technology may be used to produce a push-pull sensor wherein the pinned layer magnetization of the different bridge arms is set in opposite directions. When using the MCP technique, it is important to match the performance of the different sensor ships in the package. The different sensor chips in the package should come from the same silicon wafer, or they should be tested and sorted. The chips are then placed in the package where one is rotated 180 degrees from the other in order to produce a push-pull bridge. Although this technique is manufacturing friendly, temperature compensation is not as good; costs are higher due to package size and chip placement; it is difficult to properly align the chips at 180 degrees; it is difficult to match the performance of the two chips, such that there may be relatively large bias voltage asymmetries, etc. In summary, this easy manufacturing process brings in new problems. Exotic techniques such as local laser heating assisted magnetic reversal may also be used. In this method, GMR or MTJ wafers are initially annealed at high temperature in a strong magnetic field, which sets the magnetization of the different bridge arms in the same direction. At a later step in the process, a scanning laser beam plus reversed magnetic field is used to locally heat the wafer in the regions where the pinned layer needs to be reversed. Although it sounds easy in concept, the local laser heating method requires special equipment that is not commercially available, and development of the equipment is expensive. The process is also expensive to utilize, since it requires a long time to treat an entire wafer. Performance is also an issue, since it can be difficult to properly match other performance of the push and pull sensor arms that result in the process.
As illustrated above, there are few good options for producing low-cost MTJ or GMR sensor bridges using standard semiconductor processes.